The present invention relates to a multilayered, substantially non-oriented film comprising a base layer and at least one surface layer, to a process of producing the multilayered film and to the use of the multilayered film for wrapping food.
Films which are used for wrapping food such as vegetables, meat or fish have to meet a number of requirements, among others a good puncture resistance and a good elastic recovery. Polyvinylchloride (PVC) films meet these requirements and have been used in the past to a great extent for wrapping food. However, due to the increasing environmental concerns about the extended use of PVC films, the skilled artisans have tried to replace PVC films with other films having similar properties but causing less environmental concerns. Much research effort has been spent on ethylene homo- or copolymers or mixtures of these polymers with other polymers.
The abstract of Japanese Patent Application 61200142 discloses a composition which is composed of 80 to 99.9 weight percent of polyolefin resin containing an ethylene/acrylic copolymer as essential component and 20 to 0.1 weight percent of a water-absorbable resin of polyacrylic acids. The polyolefin is for example polyethylene, an ethylene/vinyl acetate copolymer, an ethylene/acrylic acid copolymer or polypropylene. The ethylene/acrylic copolymer is for example an ethylene/acrylic acid copolymer or an ionic copolymer thereof Evidently a monolayered film is made from this resin composition of which the essential property is said to be its excellent water-adsorbability.
The abstract of Japanese Patent Application 61083038 discloses a multilayered bag which has an inner layer of a polyolefinic resin and an anti-fogging agent. It is disclosed that each layer can be produced of a polyolefin resin such as polyethylene, polypropylene, an ethylene/vinyl acetate polymer, an ethylene/acrylic acid copolymer etc. Unfortunately, the disclosed teaching is so broad and unspecific with respect to the polymers in the multilayered film that the skilled artisan does not learn what type of polymer are to be selected in order to produce a film having the desired properties.
U.S. Pat. No. 4,277,578 relates to heat shrinkable packaging films from polyolefin blends. The blends consist of an ethylene/alpha-olefin copolymer with a density of up to 0.91 g/cm.sup.3 with a polymer (a) and/or (c). Polymer (a) is a low density polyethylene and/or a copolymer of ethylene with a vinylester, with an unsaturated aliphatic monocarboxylic acid such as acrylic acid or with its alkyl ester and polymer (c) is a crystalline polypropylene, a high density polyethylene and/or crystalline polybutene-1. U.S. Pat. No. 4,277,578 teaches that a film of poor dimensional stability is obtained when the amount of polymers (c) is below a certain level and that such a film tends to undergo deterioration by aging.
U.S. Pat. No. 4,619,859 discloses a multilayered oriented film of good sealability and stretchability having at least three layers, preferably five layers. It contains a base layer which consists of a mixture of an ethylene type polymer and a soft elastomer, a core layer produced of polypropylene and polybutene-1 and a surface layer containing an ethylene type polymer, a soft elastomer, crystalline 1,2-polybutadiene and/or a soft ionomer resin from an ethylenic copolymer. The ethylene type polymer is selected from a low density polyethylene, a copolymer of ethylene with a vinyl ester monomer, an aliphatic unsaturated monocarboxylic acid or with an alkyl ester of that monocarboxylic acid. Instead or in addition to the ethylene type polymer the base layer may contain a crystalline polypropylene or a crystalline polybutene-1 for improving the strength and the workability of the film. Unfortunately, the production of the multilayered oriented film which contains at least three different layers and wherein at least one of the layers contains a mixture of various types of polymers is labor intensive and the film structure is complicated. For example, the preparation of the mixtures by kneading and extrusion requires expensive machines.
European Patent Application 0 243 965 discloses a multilayered film useful for packaging vegetables and fruits. The film has a base layer A and at least one surface layer B containing an anti-fogging agent. The base layer A is a mixture of 1) one or more copolymers of alpha-olefins having 2 to 10 carbon atoms and 2) one or more copolymers containing monomer units selected from vinyl acetate, acrylic acid and styrene and other copolymerizable monomer units such as ethylene, propylene, acrylates, butadiene and the like. Random or block copolymers of 2 or more alpha-olefins having 2 to 10 carbon atoms are preferred for producing the surface layer(s) B. This film is designed for producing bags. Furthermore, the disclosed teaching is so broad and unspecific with respect to the polymers in the multilayered film that the skilled artisan does not learn what type of polymers are to be selected in order to produce a film which is useful for wrapping food instead of producing bags.
U.S. Pat. No. 4,418,114 discloses a stretch-wrap film comprising a layer of linear low density polyethylene (LLDPE) laminated to a branched ethylene containing polymeric layer. Specifically, the stretch-wrap film contains relatively thin outer layers of highly branched low density polyethylene and a relatively thicker LLDPE core layer.
U.S. Pat. No. 4,424,243 discloses a heat-shrinkable multilayered laminate film possessing excellent heat resistance, oil resistance and gas-barrier properties. The film is made of a core layer of a vinylidene chloride copolymer, an intermediate layer of a thermoplastic resin, such as an ethylene/vinyl acetate copolymer or 1,2-butadiene, and a surface layer of an ethylene/alpha olefin copolymer, such as LLDPE. The U.S. patent indicates that the laminate film is preferably made in tubular form. It is suitable for use in packaging by heat shrinkage meat, cheese and other fat-containing food.
European Patent Application 0 229 715 relates to multilayered heat-shrinkable films. The films are high shrinkage/low shrinkage force shrink films. The films have one or two outer layers of material selected from the group consisting of polyethylene, polypropylene, propylene/ethylene copolymers and blends thereof. The films have a core layer of material having a melting point below the temperature required to orient the material of the outer layer or layers and is selected from the group consisting of ethylene/acid copolymers and related ionomers; ethylene/acid/acrylate terpolymers and related ionomers; blends of the above with up to about 50% ethylene/vinyl acetate copolymers; blends of ethylene/ester copolymers with up to about 50% ethylene/vinyl acetate copolymers; and combinations of the above materials. The core layer constitutes about 50 to 95%, preferably about 60 to 90%, of the thickness of the overall film. Preferably, however, the combined thickness of the outer layers does not exceed about 15% of the thickness of the final film because otherwise greater stress is required to orient the film, leading to a film with a higher shrink force. The films of the European Patent Application 0 229 715 are prepared by lamination or extrusion and a subsequent orientation step. According to the disclosed methods a film is produced, quenched and then reheated to produce the final stretched or oriented film. Unfortunately, the Elmendorf tear strength of the produced oriented film according to the examples is very low (20 g/mil at the most and in nearly all cases considerably less).
Accordingly, it would be desirable to provide new multilayered films which are useful for packaging food products, in particular fresh food such as meat, fish, cheese, vegetables and fruit. In particular, it would be desirable to provide such films which do not require expensive blending techniques or equipment when manufacturing them. Furthermore, it would be desirable to provide films with good puncture resistance and deformation recovery properties. A high puncture resistance is for example important when packaging meat with bones. Good deformation recovery properties are very important for films which will be used as a packaging material for food. Typically the food is sold in self-service shops where many customers touch the packages. By touching the film it is deformed at several spots. When the food is wrapped in films having insufficient deformation recovery properties the packaged food looks unfresh after a short time and often cannot be sold anymore.
It would also be desirable to provide films which can be wrapped around the food in automatic packaging machines in particular in high speed packaging machines. In particular, it would be desirable to provide films which can be easily cut into individual samples in their cross direction but which have a high tear strength in their machine direction in order to avoid splitting of the film during the packaging process.